Use Cases for MPLS Network Action Indicators and MPLS Ancillary Data
draft-ietf-mpls-mna-usecases-15

MPLS Working Group                                               T. Saad
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                              K. Makhijani
Expires: 27 March 2025                                       Independent
                                                                 H. Song
                                                  Futurewei Technologies
                                                               G. Mirsky
                                                                Ericsson
                                                       23 September 2024

  Use Cases for MPLS Network Action Indicators and MPLS Ancillary Data
                    draft-ietf-mpls-mna-usecases-15

Abstract

   This document presents use cases that have a common feature that may
   be addressed by encoding network action indicators and associated
   ancillary data within MPLS packets.  There is community interest in
   extending the MPLS data plane to carry such indicators and ancillary
   data to address the use cases that are described in this document.

   The use cases described in this document are not an exhaustive set,
   but rather the ones that are actively discussed by members of the
   IETF MPLS, PALS, and DetNet working groups from the beginning of work
   on the MPLS Network Action until the publication of this document.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 March 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions used in this document . . . . . . . . . . . .   3
       1.2.1.  Acronyms and Abbreviations  . . . . . . . . . . . . .   3
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  No Further Fast Reroute . . . . . . . . . . . . . . . . .   4
     2.2.  Applicability of Hybrid Measurement Methods . . . . . . .   4
       2.2.1.  In-situ OAM . . . . . . . . . . . . . . . . . . . . .   5
       2.2.2.  Alternate Marking Method  . . . . . . . . . . . . . .   5
     2.3.  Network Slicing . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  NSH-based Service Function Chaining . . . . . . . . . . .   6
     2.5.  Network Programming . . . . . . . . . . . . . . . . . . .   7
   3.  Co-existence with the Existing MPLS Services Using Post-Stack
           Headers . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Co-existence of the MNA Use Cases . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Use Cases for Continued Discussion . . . . . . . . .  13
     A.1.  Generic Delivery Functions  . . . . . . . . . . . . . . .  13
     A.2.  Delay Budgets for Time-Bound Applications . . . . . . . .  13
     A.3.  Stack-Based Methods for Latency Control . . . . . . . . .  14
   Contributors' Addresses . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   This document describes use cases that introduce functions that
   require special processing by forwarding hardware.  The current state
   of the art requires allocating a new special-purpose label (SPL)
   [RFC3032] or extended special-purpose label (eSPL).  SPLs are a very
   limited resource, while eSPL requires an extra Label Stack Entry per
   Network Action, which is expensive.  Therefore, an MPLS Network
   Action (MNA) [RFC9613] approach was proposed to extend the MPLS

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   architecture.  MNA is expected to enable functions that may require
   carrying additional ancillary data within the MPLS packets, as well
   as a means to indicate the ancillary data is present and a specific
   action needs to be performed on the packet.

   This document lists various use cases that could benefit extensively
   from the MNA framework [I-D.ietf-mpls-mna-fwk].  Supporting a
   solution of the general MNA framework provides a common foundation
   for future network actions that can be exercised in the MPLS data
   plane.

1.1.  Terminology

   The following terminology is used in the document:

   RFC 9543 Network Slice
      is interpreted as defined in [RFC9543].  Furthermore, this
      document uses "network slice" interchangeably as a shorter version
      of the RFC 9543 Network Slice term.

   The MPLS Ancillary Data is classified as:
      *  residing within the MPLS label stack and referred to as In-
         Stack Data, and

      *  residing after the Bottom of Stack (BoS) and referred to as
         Post-Stack Data.

1.2.  Conventions used in this document

1.2.1.  Acronyms and Abbreviations

      MNA: MPLS Network Action

      DEX: Direct Export

      I2E: Ingress to Edge

      HbH: Hop by Hop

      PW: Pseudowire

      BoS: Bottom of Stack

      ToS: Top of Stack

      NSH: Network Service Header

      FRR: Fast Reroute

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      IOAM: In-situ Operations, Administration, and Maintenance

      G-ACh: Generic Associated Channel

      LSP: Label Switched Path

      LSR: Label Switch Router

      NRP: Network Resource Partition

      SPL: Special Purpose Label

      eSPL: extended Special Purpose Label

      AMM: Alternative Marking Method

2.  Use Cases

2.1.  No Further Fast Reroute

   MPLS Fast Reroute [RFC4090], [RFC5286], [RFC7490], and
   [I-D.ietf-rtgwg-segment-routing-ti-lfa] is a useful and widely
   deployed tool for minimizing packet loss in the case of a link or
   node failure.

   Several cases exist where, once a Fast Reroute (FRR) has taken place
   in an MPLS network and a packet is rerouted away from the failure, a
   second FRR impacts the same packet on another node and may result in
   traffic disruption.

   In such a case, the packet impacted by multiple FRR events may
   continue to loop between the label switch routers (LSRs) that
   activated FRR until the packet's TTL expires.  That can lead to link
   congestion and further packet loss.  To avoid that situation, packets
   that FRR has redirected will be marked using MNA to preclude further
   FRR processing.

2.2.  Applicability of Hybrid Measurement Methods

   MNA can be used to carry information essential for collecting
   operational information and measuring various performance metrics
   that reflect the experience of the packet marked by MNA.  Optionally,
   the operational state and telemetry information collected on the LSR
   may be transported using MNA techniques.

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2.2.1.  In-situ OAM

   In-situ Operations, Administration, and Maintenance (IOAM), defined
   in [RFC9197] and [RFC9326], might be used to collect operational and
   telemetry information while a packet traverses a particular path in a
   network domain.

   IOAM can run in two modes: Ingress to Edge (I2E) and Hop by Hop
   (HbH).  In I2E mode, only the encapsulating and decapsulating nodes
   will process IOAM data fields.  In HbH mode, the encapsulating and
   decapsulating nodes and intermediate IOAM-capable nodes process IOAM
   data fields.  The IOAM data fields, defined in [RFC9197], can be used
   to derive the operational state of the network experienced by the
   packet with the IOAM Header that traversed the path through the IOAM
   domain.

   Several IOAM Option-Types have been defined:

   *  Pre-allocated Trace

   *  Incremental Trace

   *  Edge-to-Edge

   *  Proof-of-Transit

   *  Direct Export (DEX)

   With all IOAM Option-Types except for the Direct Export (DEX), the
   collected information is transported in the trigger IOAM packet.  In
   the IOAM DEX Option [RFC9326], the operational state and telemetry
   information are collected according to a specified profile and
   exported in a manner and format defined by a local policy.  In IOAM
   DEX, the user data packet is only used to trigger the IOAM data to be
   directly exported or locally aggregated without being carried in the
   IOAM trigger packets.

2.2.2.  Alternate Marking Method

   The Alternate Marking Method (AMM), defined in [RFC9341] and
   [RFC9342]) is an example of a hybrid performance measurement method
   ([RFC7799]) that can be used in the MPLS network to measure packet
   loss and packet delay performance metrics.  [RFC8957] defined the
   Synonymous Flow Label framework to realize AMM in the MPLS network.
   The MNA is an alternative mechanism that can be used to support AMM
   in the MPLS network.

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2.3.  Network Slicing

   An RFC 9543 Network Slice service ([RFC9543]) provides connectivity
   coupled with network resource commitments and is expressed in terms
   of one or more connectivity constructs.  Section 5 of
   [I-D.ietf-teas-ns-ip-mpls] defines a Network Resource Partition (NRP)
   Policy as a policy construct that enables the instantiation of
   mechanisms to support one or more network slice services.  The
   packets associated with an NRP may carry a marking in their network
   layer header to identify this association, referred to as an NRP
   Selector.  The NRP Selector maps a packet to the associated network
   resources and provides the corresponding forwarding treatment onto
   the packet.

   A router that requires the forwarding of a packet that belongs to an
   NRP may have to decide on the forwarding action to take based on
   selected next-hop(s), and the forwarding treatment (e.g., scheduling
   and drop policy) to enforce based on the associated per-hop behavior.

   In this case, routers that forward traffic over a physical link
   shared by multiple NRPs need to identify the NRP to which the packet
   belongs to enforce their respective forwarding actions and
   treatments.

   MNA technologies can signal actions for MPLS packets and carry data
   essential for these actions.  For example, MNA can carry the NRP
   Selector [I-D.ietf-teas-ns-ip-mpls] in MPLS packets.

2.4.  NSH-based Service Function Chaining

   [RFC8595] describes how Service Function Chaining can be realized in
   an MPLS network by emulating the Network Service Header (NSH)
   [RFC8300] using only MPLS label stack elements.

   The approach in [RFC8595] introduces some limitations discussed in
   [I-D.lm-mpls-sfc-path-verification].  However, that approach can
   benefit from the framework introduced with MNA in
   [I-D.ietf-mpls-mna-fwk].

   MNA can be used to extend NSH emulation using MPLS labels [RFC8595]
   to support the functionality of NSH Context Headers, whether fixed or
   variable-length.  For example, MNA could support Flow ID [RFC9263]
   that may be used for load-balancing among Service Function Forwarders
   and/or the Service Functions within the same Service Function Path.

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2.5.  Network Programming

   In Segment Routing (SR), an ingress node steers a packet through an
   ordered list of instructions called "segments".  Each of these
   instructions represents a function to be called at a specific
   location in the network.  A function is locally defined on the node
   where it is executed and may range from simply moving forward in the
   segment list to any complex user-defined behavior.

   Network Programming combines SR functions to achieve a networking
   objective beyond mere packet routing.

   Encoding a pointer to a function and its arguments within an MPLS
   packet transport header may be desirable.  MNA can be used to encode
   the FUNC::ARGs to support the functional equivalent of FUNC::ARG in
   SRv6 as described in [RFC8986].

3.  Co-existence with the Existing MPLS Services Using Post-Stack
    Headers

   Several services can be transported over MPLS networks today.  These
   include providing Layer-3 (L3) connectivity (e.g., for unicast and
   multicast L3 services), and Layer-2 (L2) connectivity (e.g., for
   unicast Pseudowires (PWs), multicast E-Tree, and broadcast E-LAN L2
   services).  In those cases, the user service traffic is encapsulated
   as the payload in MPLS packets.

   For L2 service traffic, it is possible to use Control Word (CW)
   [RFC4385] and [RFC5085] immediately after the MPLS header to
   disambiguate the type of MPLS payload, prevent possible packet
   misordering, and allow for fragmentation.  In this case, the first
   nibble the data that immediately follows after the MPLS BoS is set to
   0b0000 to identify the presence of PW CW.

   In addition to providing connectivity to user traffic, MPLS may also
   transport OAM data (e.g., over MPLS Generic Associated Channels
   (G-AChs) [RFC5586]).  In this case, the first nibble of the data that
   immediately follows after the MPLS BoS is set to 0b0001.  It
   indicates the presence of a control channel associated with a PW,
   LSP, or Section.

   Bit Index Explicit Replication (BIER) [RFC8296] traffic can also be
   encapsulated over MPLS.  In this case, BIER has defined 0b0101 as the
   value for the first nibble in the data that immediately appears after
   the bottom of the label stack for any BIER-encapsulated packet over
   MPLS.

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   For pseudowires, the Generic Associated Channel [RFC7212] uses the
   first four bits of the PW control word to provide the initial
   discrimination between data packets and packets belonging to the
   associated channel, as described in [RFC4385].

   MPLS can be used as the data plane for DetNet [RFC8655].  The DetNet
   sub-layers, forwarding, and service are realized using the MPLS label
   stack, the DetNet Control Word [RFC8964], and the DetNet Associated
   Channel Header [RFC9546].

   MNA-based solutions for the use cases described in this document and
   proposed in the future are expected to allow for coexistence and
   backward compatibility with all existing MPLS services.

4.  Co-existence of the MNA Use Cases

   Two or more of the discussed cases may co-exist in the same packet.
   That may require the presence of multiple ancillary data (whether In-
   stack or Post-stack ancillary data) to be present in the same MPLS
   packet.

   For example, IOAM may provide essential functions along with network
   slicing to help ensure that critical network slice SLOs are being met
   by the network provider.  In this case, IOAM can collect key
   performance measurement parameters of network slice traffic flow as
   it traverses the transport network.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   Section 7 of "MPLS Network Action (MNA) Framework",
   [I-D.ietf-mpls-mna-fwk] outlines security considerations for non-
   protocol specifying documents.  The authors have verified that these
   considerations are fully applicable to this document.

   In-depth security analysis for each specific use case is beyond the
   scope of this document and will be addressed in future solution
   documents.  It is strongly recommended that these solution documents
   undergo security expert review early in their development, ideally
   during the Working Group Last Call phase.

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7.  Acknowledgement

   The authors gratefully acknowledge the input of the members of the
   MPLS Open Design Team.  Also, the authors sincerely thank Loa
   Andersson, Xiao Min, Jie Dong, and Yaron Sheffer. for their
   thoughtful suggestions and help in improving the document.

8.  References

8.1.  Normative References

   [I-D.ietf-mpls-mna-fwk]
              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions (MNA) Framework", Work in Progress,
              Internet-Draft, draft-ietf-mpls-mna-fwk-10, 6 August 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-fwk-10>.

8.2.  Informative References

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              17, 5 July 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rtgwg-segment-routing-ti-lfa-17>.

   [I-D.ietf-teas-ns-ip-mpls]
              Saad, T., Beeram, V. P., Dong, J., Wen, B., Ceccarelli,
              D., Halpern, J. M., Peng, S., Chen, R., Liu, X.,
              Contreras, L. M., Rokui, R., and L. Jalil, "Realizing
              Network Slices in IP/MPLS Networks", Work in Progress,
              Internet-Draft, draft-ietf-teas-ns-ip-mpls-04, 28 May
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              teas-ns-ip-mpls-04>.

   [I-D.lm-mpls-sfc-path-verification]
              Liu, Y. and G. Mirsky, "MPLS-based Service Function
              Path(SFP) Consistency Verification", Work in Progress,
              Internet-Draft, draft-lm-mpls-sfc-path-verification-03, 11
              June 2022, <https://datatracker.ietf.org/doc/html/draft-
              lm-mpls-sfc-path-verification-03>.

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   [I-D.stein-srtsn]
              Stein, Y. J., "Segment Routed Time Sensitive Networking",
              Work in Progress, Internet-Draft, draft-stein-srtsn-01, 29
              August 2021, <https://datatracker.ietf.org/doc/html/draft-
              stein-srtsn-01>.

   [I-D.zzhang-intarea-generic-delivery-functions]
              Zhang, Z. J., Bonica, R., Kompella, K., and G. Mirsky,
              "Generic Delivery Functions", Work in Progress, Internet-
              Draft, draft-zzhang-intarea-generic-delivery-functions-03,
              11 July 2022, <https://datatracker.ietf.org/doc/html/
              draft-zzhang-intarea-generic-delivery-functions-03>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <https://www.rfc-editor.org/info/rfc5085>.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [RFC7212]  Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
              Associated Channel (G-ACh) Advertisement Protocol",
              RFC 7212, DOI 10.17487/RFC7212, June 2014,
              <https://www.rfc-editor.org/info/rfc7212>.

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   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [RFC8300]  Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
              "Network Service Header (NSH)", RFC 8300,
              DOI 10.17487/RFC8300, January 2018,
              <https://www.rfc-editor.org/info/rfc8300>.

   [RFC8595]  Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based
              Forwarding Plane for Service Function Chaining", RFC 8595,
              DOI 10.17487/RFC8595, June 2019,
              <https://www.rfc-editor.org/info/rfc8595>.

   [RFC8655]  Finn, N., Thubert, P., Varga, B., and J. Farkas,
              "Deterministic Networking Architecture", RFC 8655,
              DOI 10.17487/RFC8655, October 2019,
              <https://www.rfc-editor.org/info/rfc8655>.

   [RFC8957]  Bryant, S., Chen, M., Swallow, G., Sivabalan, S., and G.
              Mirsky, "Synonymous Flow Label Framework", RFC 8957,
              DOI 10.17487/RFC8957, January 2021,
              <https://www.rfc-editor.org/info/rfc8957>.

   [RFC8964]  Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
              S., and J. Korhonen, "Deterministic Networking (DetNet)
              Data Plane: MPLS", RFC 8964, DOI 10.17487/RFC8964, January
              2021, <https://www.rfc-editor.org/info/rfc8964>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

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   [RFC9197]  Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In Situ Operations, Administration,
              and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
              May 2022, <https://www.rfc-editor.org/info/rfc9197>.

   [RFC9263]  Wei, Y., Ed., Elzur, U., Majee, S., Pignataro, C., and D.
              Eastlake 3rd, "Network Service Header (NSH) Metadata Type
              2 Variable-Length Context Headers", RFC 9263,
              DOI 10.17487/RFC9263, August 2022,
              <https://www.rfc-editor.org/info/rfc9263>.

   [RFC9326]  Song, H., Gafni, B., Brockners, F., Bhandari, S., and T.
              Mizrahi, "In Situ Operations, Administration, and
              Maintenance (IOAM) Direct Exporting", RFC 9326,
              DOI 10.17487/RFC9326, November 2022,
              <https://www.rfc-editor.org/info/rfc9326>.

   [RFC9341]  Fioccola, G., Ed., Cociglio, M., Mirsky, G., Mizrahi, T.,
              and T. Zhou, "Alternate-Marking Method", RFC 9341,
              DOI 10.17487/RFC9341, December 2022,
              <https://www.rfc-editor.org/info/rfc9341>.

   [RFC9342]  Fioccola, G., Ed., Cociglio, M., Sapio, A., Sisto, R., and
              T. Zhou, "Clustered Alternate-Marking Method", RFC 9342,
              DOI 10.17487/RFC9342, December 2022,
              <https://www.rfc-editor.org/info/rfc9342>.

   [RFC9543]  Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
              Makhijani, K., Contreras, L., and J. Tantsura, "A
              Framework for Network Slices in Networks Built from IETF
              Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
              <https://www.rfc-editor.org/info/rfc9543>.

   [RFC9546]  Mirsky, G., Chen, M., and B. Varga, "Operations,
              Administration, and Maintenance (OAM) for Deterministic
              Networking (DetNet) with the MPLS Data Plane", RFC 9546,
              DOI 10.17487/RFC9546, February 2024,
              <https://www.rfc-editor.org/info/rfc9546>.

   [RFC9613]  Bocci, M., Ed., Bryant, S., and J. Drake, "Requirements
              for Solutions that Support MPLS Network Actions (MNAs)",
              RFC 9613, DOI 10.17487/RFC9613, August 2024,
              <https://www.rfc-editor.org/info/rfc9613>.

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Appendix A.  Use Cases for Continued Discussion

   Several use cases for which MNA can provide a viable solution have
   been discussed.  The discussion of these aspirational cases is
   ongoing at the time of publication of the document.

A.1.  Generic Delivery Functions

   The Generic Delivery Functions (GDFs), defined in
   [I-D.zzhang-intarea-generic-delivery-functions], provide a new
   mechanism to support functions analogous to those supported through
   the IPv6 Extension Headers mechanism.  For example, GDF can support
   fragmentation/reassembly functionality in the MPLS network by using
   the Generic Fragmentation Header.  MNA can support GDF by placing a
   GDF header in an MPLS packet within the Post-Stack Data block
   [I-D.ietf-mpls-mna-fwk].  Multiple GDF headers can also be present in
   the same MPLS packet organized as a list of headers.

A.2.  Delay Budgets for Time-Bound Applications

   The routers in a network can perform two distinct functions on
   incoming packets, namely forwarding (where the packet should be sent)
   and scheduling (when the packet should be sent).  IEEE-802.1 Time
   Sensitive Networking (TSN) and Deterministic Networking provide
   several mechanisms for scheduling under the assumption that routers
   are time-synchronized.  The most effective mechanisms for delay
   minimization involve per-flow resource allocation.

   Segment Routing (SR) is a forwarding paradigm that allows encoding
   forwarding instructions in the packet in a stack data structure
   rather than being programmed into the routers.  The SR instructions
   are contained within a packet in the form of a First-in, First-out
   stack dictating the forwarding decisions of successive routers.
   Segment routing may be used to choose a path sufficiently short to be
   capable of providing a bounded end-to-end latency but does not
   influence the queueing of individual packets in each router along
   that path.

   When carried over the MPLS data plane, a solution is required to
   enable the delivery of such packets that can be delivered to their
   final destination within a given time budget.  One approach to
   address this use case in SR-MPLS was described in [I-D.stein-srtsn].

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A.3.  Stack-Based Methods for Latency Control

   One efficient data structure for inserting local deadlines into the
   headers is a "stack", similar to that used in Segment Routing to
   carry forwarding instructions.  The number of deadline values in the
   stack equals the number of routers the packet needs to traverse in
   the network, and each deadline value corresponds to a specific
   router.  The Top-of-Stack (ToS) corresponds to the first router's
   deadline, while the MPLS BoS refers to the last.  All local deadlines
   in the stack are later or equal to the current time (upon which all
   routers agree), and times closer to the ToS are always earlier or
   equal to times closer to the MPLS BoS.

   The ingress router inserts the deadline stack into the packet
   headers; no other router needs to know the time-bound flows'
   requirements.  Hence, admitting a new flow only requires updating the
   ingress router's information base.

   MPLS LSRs that expose the ToS label can also inspect the associated
   "deadline" carried in the packet (either in the MPLS stack as In-
   Stack Data or after BoS as Post-Stack Data).

Contributors' Addresses

   Loa Anderssen
   Bronze Dragon Consulting
   Email: [email protected]

Authors' Addresses

   Tarek Saad
   Cisco Systems, Inc.
   Email: [email protected]

   Kiran Makhijani
   Independent
   Email: [email protected]

   Haoyu Song
   Futurewei Technologies
   Email: [email protected]

   Greg Mirsky
   Ericsson

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   Email: [email protected]

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